1. Field of the Invention
The present invention relates to image pickup apparatuses, image pickup methods, programs, and recording media. In particular, the present invention relates to an image pickup apparatus, an image pickup method, a program, and a recording medium which are preferably applied to processing associated with multiple exposure.
2. Description of the Related Art
There is a photographing technique called multiple exposure, for use in cameras for picking up images. In multiple exposure, a single frame is exposed multiple times, to light corresponding to a single or a plurality of image objects, and a plurality of images are superimposed to create a combined image. In particular, in multiple exposure photography with a long exposure time using a slow shutter or a bulb shutter, multiple exposure realizes photographs of night scenes, fireworks, or the like, with highly artistic effects.
In film cameras using a film as a recording medium, multiple exposure photography is realized by operating the shutter multiple times without winding the film on. In digital image pickup apparatuses such as digital still cameras and digital video cameras which have recently been in widespread use, multiple exposure is not realized using the same processing as in employed in film cameras. However, there has been a necessity for a digital image pickup apparatus having a multiple exposure function, similarly to film cameras, and such a digital image pickup apparatus has gradually become commercially available.
To realize a multiple exposure function in a digital image pickup apparatus, it is necessary to save a plurality of images in order to generate a combined image. This saving operation necessitates a large-capacity storage device such as a memory for saving a plurality of images, which leads to increases in the size and cost of the image pickup apparatus. In particular, with the recent trend of increase in the number of pixels and downsizing of image pickup devices, such large-capacity storage devices may cause an increase in size of the apparatus.
In addition, an increase in the memory capacity may increase the memory bandwidth. An increase in the memory bandwidth may cause a conflict with other functions using the memory, which would significantly affect the processing performance of the entire system of an image pickup apparatus, such as the photographing speed.
Further, in multiple exposure with a long exposure time, the sensitivity and dynamic range of an image pickup device may be decreased by an increase in a black signal level and generation of fixed pattern noise which are associated with a so-called dark current in a solid-state image pickup device. Such a dark current is caused by electron-hole pairs generated as a result of thermal excitation of a semiconductor. The effect of a dark current increases with increasing temperature and increasing storage time of electric charge.
Japanese Unexamined Patent Application Publication No. 2001-28726 discloses a multiple-exposure photography method. In this method, a main image signal is generated by extracting main object parts from a plurality of images and recording an image corresponding to a difference between two consecutive main image signals. Accordingly, this makes it possible to reduce a memory capacity necessary for generating a combined image.